In this type of imaging lens, the optical length, which is defined as the distance from an entrance plane on the object side of the imaging lens to an imaging plane (the imaging surface of the CCD or the like) must be short. In other words, measures must be taken during lens design to reduce the ratio of the optical length to the combined focal length of the imaging lens. Hereafter, an imaging lens in which the optical length is short and the ratio of the optical length to the focal length is small will occasionally be referred to as a compact lens.
Taking a portable telephone as an example, at least the optical length must be shorter than the thickness of the portable telephone main body. On the other hand, the back focus, which is defined as the distance from an exit plane on the image side of the imaging lens to the imaging surface is preferably as long as possible. In other words, measures must be taken during lens design to make the ratio of the back focus to the focal length as large as possible so that components such as a filter and cover glass can be inserted between the imaging lens and the imaging surface.
In addition to the above, demands have been made for an imaging lens in which various kinds of aberration are corrected in accordance with the density of the imaging elements (also known as “pixels”) to a small enough degree that image distortion is not visible to the human eye. In other words, various kinds of aberration must be corrected favorably, and hereafter, an image in which various kinds of aberration have been corrected favorably will occasionally be referred to as a “favorable image”.
As will be described below, imaging lenses having a three-layer structure, which are suitable for use in an imaging apparatus such as a portable computer or videophone apparatus employing a CCD, CMOS, or other solid state imaging element, have been disclosed. These lenses all secure a wide viewing angle, and are small and lightweight.
Of these lenses, an imaging lens which produces images with favorably corrected aberrations while securing a wide viewing angle has been disclosed as a first three-layer lens (see Patent Document 1, for example).
However, of the three lenses arranged in sequence from the object side as a first lens, a second lens, and a third lens, the first lens is a meniscus lens having a positive refractive power and a convex surface facing the image side, the second lens is a meniscus lens having a negative refractive power and a convex surface facing the object side, and the third lens is a convex lens having a positive refractive power, and hence the optical length is too long in relation to the back focus. As a result, a compact lens cannot be provided.
Imaging lenses in which various aberrations are favorably corrected and a short focus is realized while securing a wide viewing angle have been disclosed respectively as second through fourth three-layer lenses (see Patent Document 2, Patent Document 3 and Patent Document 4, for example).
However, similarly to the imaging lens described above, the refractive power of the three lenses of these imaging lenses, constituted by first, second, and third lenses arranged in sequence from the object side, is positive in the first lens, negative in the second lens, and positive in the third lens. Hence, although these imaging lenses have a short combined focal length, the back focus is long in relation to the combined focal length, and therefore the optical length is also too long. In addition, the lenses use glass materials and are therefore expensive.
An imaging lens which uses aspherical lenses and is reduced in size by appropriately setting the power distribution and surface shape of the lenses has been disclosed as a fifth three-layer imaging lens (see Patent Document 5, for example)
However, the refractive power of the three lenses of this imaging lens, constituted by first, second, and third lenses arranged in succession from the object side, is negative in the first lens, positive in the second lens, and negative in the third lens. As a result, the imaging lens has a long optical length in relation to the combined focal length. In addition, the lenses use glass materials and are therefore expensive.
An imaging lens in which a pair of meniscus lenses whose concave surfaces face each other are constituted by plastic lenses each having at least one aspherical surface, and in which the entire lens system has a three-layer structure, has been disclosed as a sixth three-layer imaging lens. This lens achieves compactness and low cost, and is capable of suppressing focus movement due to temperature change with ease (see Patent Document 6, for example).
However, the refractive power of the three lenses in this imaging lens, which are arranged as first, second, and third lenses in succession from the object side, is weak in the first lens, weak in the second lens, and positive in the third lens. Hence the refractive power of the first lens and second lens cannot be fully compensated for by the third lens alone. As a result, the back focus lengthens in relation to the combined focal length, causing an increase in the optical length. Furthermore, the third lens uses a glass material, and therefore cost reduction is incomplete.
A telephoto-type lens system in which the entire lens system is divided into front and rear groups, the front group having a positive refractive power and the rear group having a negative refractive power, has been disclosed as a seventh three-layer lens. This lens system has a short optical length and is low in cost (see Patent Document 7, for example).
However, the refractive power of the three lenses in this lens system, which are arranged as first, second, and third lenses in succession from the object side, is negative in the first lens, positive in the second lens, and negative in the third lens, and the interval between the second lens and third lens is great. As a result, the optical length is long in relation to the combined focal length, and the aperture of the third lens is large. Accordingly, this lens system is unsuitable for installation in an image input device of a portable telephone or personal computer, a digital camera, a CCD camera used for monitoring purposes, a surveying apparatus, and so on.
An imaging lens comprising, in succession from the object side, two positive lenses, and a negative lens having a concave surface facing the image side, two aspherical surfaces, and a negative power which gradually weakens from the center of the lens toward the periphery so as to become a positive power on the periphery, has been disclosed as an eighth three-layer lens (see Patent Document 8, for example).
However, in this lens system, the negative power of a lens corresponding to a third lens L3 gradually weakens from the center of the lens toward the periphery, and the point at which the negative power becomes positive power exists within a range of 0.7 times to 1.0 times the effective aperture of the lens from the center of the lens. In lenses disclosed as embodiments, the positions at which the negative power becomes positive power are set at 0.96 times and 0.97 times the effective aperture of the lens from the center of the lens, respectively, and hence the positive power is set substantially on the peripheral portion of the lens.
By setting the position at which negative power turns to positive power on the peripheral portion of the lens, light entering the peripheral portion and the vicinity of the intersection between the optical axis and imaging surface of the lens enters the imaging element at an angle of incidence which is almost a right angle, but in an intermediate position between the peripheral portion of the lens and the intersection between the optical axis and imaging surface of the lens, the angle of incidence of the light entering the imaging element is greatly removed from a right angle. Hence, in intermediate positions relative to the peripheral portion of the lens, which occupies an important part of the image, the angle of incidence of the light is greatly removed from a right angle, and as a result, the light enters the imaging element from a diagonal direction to the imaging element, leading to an increase in the amount of reflection on the entrance plane and a reduction in the amount of optical energy reaching the photoelectric conversion surface of the imaging element. Accordingly, the image darkens in this part.
An imaging lens comprising an aperture diaphragm, a biconvex first lens having a positive refractive power, a second lens having a negative refractive power and a concave surface facing the object side, and a third lens having a meniscus shape and a convex surface facing the object side, which are arranged in sequence from the object side, has been disclosed as a ninth three-layer lens (see Patent Document 9, for example).
This lens system is designed to obtain favorable images when an aperture diaphragm is disposed on the object side of the first lens. By disposing the aperture diaphragm on the object side of the first lens, the entrance pupil can be formed near the object, and as a result, principal rays can be caused to enter at a near-perpendicular angle to the image surface. When the principal rays enter the image surface diagonally, the amount of light entering the pixels (imaging elements) disposed on the image surface decreases, leading to shading in which the image darkens in the peripheral portion of the image surface.
This problem is caused when a light ray enters an imaging element from a diagonal direction to the imaging element, leading to an increase in the amount of reflection on the surface of the imaging element and a reduction in the amount of optical energy reaching the photoelectric conversion surface of the imaging element. By disposing an aperture diaphragm on the object side of the first lens, an imaging lens in which shading is unlikely to occur can be designed.
In a lens system designed on the basis of this design principle, a further diaphragm may be disposed between the first lens and second lens to prevent flare, which is a phenomenon in which the contrast of the image decreases, or smear, which is a phenomenon in which the image runs. When a diaphragm is disposed between the first lens and second lens, the principal rays which enter at a large angle of incidence in relation to the optical axis of the imaging lens, from among the principal rays that pass through the aperture diaphragm, are blocked by the second diaphragm. As a result, a part of the principal rays is blocked rather than the stray light which causes flare, smear, and other phenomena leading to a reduction in image quality, and hence in certain cases the amount of light on the periphery of the image may decrease, causing the peripheral portion of the image to darken.
Also in this lens system, the lens corresponding to the third lens is a meniscus lens, and hence the back focus is short in relation to the optical length. Therefore, when the back focus is increased so that a component such as a filter or cover glass can be inserted between the imaging lens and the imaging surface, the optical length also increases, and as a result, the lens system itself becomes too large.
An imaging lens comprising, in sequence from the object side, a first lens having a positive refractive power and a convex surface on the object side, a diaphragm, a second lens made of a plastic material and having at least one aspherical surface, a positive or negative refractive power, and a concave surface facing the object side, and a third lens having a positive refractive power, two aspherical surfaces, and a convex surface facing the object side, has been disclosed as a tenth three-layer lens (see Patent Document 10, for example).
The tenth three-layer lens is designed on the assumption that a diaphragm is set between the first lens and second lens, and this diaphragm caused to function as an aperture diaphragm so that favorable images can be obtained. Hence, when a shutter or the like is disposed on the object side of the first lens, the entrance aperture of the lens is narrowed by the shutter or the like. In other words, the shutter or the like functions essentially as a diaphragm such that a part of the principal rays entering the diaphragm is blocked. The principal rays which enter at a large angle in relation to the optical axis of the lens are the rays which form the peripheral portion of the image, and since these rays are blocked by the shutter or the like disposed on the object side of the first lens, the peripheral parts of the image may darken.
In this lens system, as in the ninth three-layer lens described above, the lens corresponding to the third lens L3 is a meniscus lens. Hence, in this lens system, as in the ninth three-layer lens, an increase in the back focus leads to an increase in the optical length, and as a result, the lens system itself becomes too large.
An imaging lens comprising, in sequence from the object side, a first lens made of a glass material and having a positive refractive power, the object-side surface of which is a convex surface, a diaphragm, a second lens made of a plastic material and having at least one aspherical surface, a positive refractive power, and a concave surface facing the object side, and a third lens made of a plastic material and having a negative refractive power, two aspherical surfaces, and a convex surface facing the object side, has been disclosed as an eleventh three-layer lens (see Patent Document 11, for example).
The eleventh three-layer lens has an identical basic structure to the tenth three-layer lens, and hence displays similar problems to those of the tenth three-layer lens.
An imaging lens comprising, in sequence from the object side, a biconvex first lens having a positive refractive power and at least one aspherical surface, a diaphragm, a second lens having at least one aspherical surface, a positive refractive power, a concave surface facing the object side, and a meniscus shape, and a third lens made of a plastic material and having a positive or negative refractive power and two aspherical surfaces, the object-side surface of which is a convex surface, has been disclosed as a twelfth three-layer lens (see Patent Document 12, for example).
The twelfth three-layer lens has an identical basic structure to the tenth and eleventh three-layer lenses described above, and hence displays similar problems to those of the tenth and eleventh three-layer lenses.
An imaging lens comprising, in sequence from the object side, a first lens having a mainly positive refractive power and a convex surface facing the object side, a second lens having a meniscus shape, a negative refractive power, and a convex surface facing the image side, and a third lens having a positive refractive power and a convex surface facing the object side, has been disclosed as a thirteenth three-layer lens. An imaging lens in which a diaphragm is disposed on the object side of the first lens and an imaging lens in which a diaphragm is disposed between the first lens and second lens is also disclosed (see Patent Document 13, for example).
More specifically, an imaging lens designed to obtain favorable images by having a diaphragm disposed on the object side of the first lens function as an aperture diaphragm, and an imaging lens designed to obtain favorable images by having a diaphragm disposed between the first lens and second lens function as an aperture diaphragm, are disclosed.
When another diaphragm is disposed between the first lens and second lens of an imaging lens that is designed to obtain favorable images by having a diaphragm disposed on the object side of the first lens function as an aperture diaphragm, the principal rays which enter the imaging lens at a large angle of incidence in relation to the optical axis of the imaging lens, from among the principal rays which pass through the aperture diaphragm, are blocked by the additional diaphragm. Similarly, when another diaphragm is disposed on the object side of the first lens of an imaging lens that is designed to obtain favorable images by having a diaphragm disposed between the first lens and second lens function as an aperture diaphragm, the principal rays which enter the imaging lens at a large angle of incidence in relation to the optical axis of the imaging lens, from among the principal rays which pass through the aperture diaphragm, are blocked by the additional diaphragm.
Hence, as described above, a part of the principal rays is blocked rather than the stray light which causes flare, smear, or other phenomena leading to a reduction in image quality, and therefore in certain cases the amount of light on the periphery of the image may decrease, causing the peripheral portion of the image to darken.
Patent Document 13 discloses an embodiment in which the imaging lens having the aperture diaphragm on the object side of the first lens and the imaging lens having the aperture diaphragm between the first lens and second lens are designed individually and independently. In other words, the shape and arrangement of the first through third lenses are designed in accordance with the placement position of the aperture diaphragm so that favorable images are obtained in each imaging lens. Accordingly, an imaging lens in which a diaphragm is disposed on the object side of the first lens and a further diaphragm serving as an aperture diaphragm is disposed between the first lens and second lens is not disclosed. In other words, an imaging lens which comprises both an aperture diaphragm for securing the entrance pupil position and a diaphragm for preventing flare or smear in order to improve the lens performance is not disclosed.
Furthermore, in the thirteenth three-layer lens, similarly to the ninth three-layer lens, the lens corresponding to a third lens L3 is a meniscus lens. Hence in this lens system, as in the ninth three-layer lens, a long back focus leads to a long optical length, and therefore the lens system itself becomes too large.    Patent Document 1: Japanese Unexamined Patent Application Publication 2001-075006    Patent Document 2: Japanese Unexamined Patent Application Publication 2003-149548    Patent Document 3: Japanese Unexamined Patent Application Publication 2002-221659    Patent Document 4: Japanese Unexamined Patent Application Publication 2002-244030    Patent Document 5: Japanese Unexamined Patent Application Publication 2003-149545    Patent Document 6: Japanese Unexamined patent Application Publication H10-301022    Patent Document 7: Japanese Unexamined Patent Application Publication H10-301021    Patent Document 8: Japanese Unexamined Patent Application Publication 2003-322792    Patent Document 9: Japanese Unexamined Patent Application Publication 2004-4566    Patent Document 10: Japanese Unexamined Patent Application Publication 2004-302058    Patent Document 11: Japanese Unexamined Patent Application Publication 2004-302059    Patent Document 12: Japanese Unexamined Patent Application Publication 2004-302060    Patent Document 13: Japanese Unexamined Patent Application Publication 2005-4045